The removal of dimethyl ether from a hydrocarbon mixture is difficult and costly. Distillation will result in the unacceptable loss of desired hydrocarbons, requiring high energy usage, in the volatility range of the dimethyl ether, while extraction with water is inefficient and costly because of the copious amounts of water needed and the large number of equilibrium stages required. The removal of dimethyl ether from a hydrocarbon mixture can be a serious problem commercially. For example, a refinery stream containing hydrocarbons having from 3 to 6 carbon atoms, including isobutylene, can be treated with methanol in the presence of an acid catalyst to cause a reaction between the olefin and the alcohol to form an alkyl ether, such as methyl tertiary butyl ether (MTBE). This can be carried out, for example, in a catalytic distillation column, wherein the feed is introduced as a side stream into the column containing the acid catalyst, a substantially pure product MTBE is continuously removed from the base of the column and a raffinate stream containing the remainder of the contents thereof, including dimethyl ether, is removed overhead. MTBE is used as an environmentally acceptable high octane enhancer for motor gasoline.
The overhead stream defined above contains hydrocarbons, including unreacted olefins, some unreacted methanol, water and dimethyl ether, formed from the dehydration of methanol, and can be charged to an alkylation plant wherein olefins can be reacted with an isoparaffin, such as isobutane, in the presence of an alkylation catalyst to form additional high octane gasoline components. However, when the alkylation catalyst is hydrogen fluoride, the presence of methanol, water and dimethyl ether in the feed to the alkylation reactor is highly detrimental, since they adversely affect the catalytic activity of the hydrogen fluoride catalyst, increasing acid loss and significantly lowering the octane value of the alkylate. The removal of water and methanol from the alkylation feed is not difficult. For example, the alkylation feed can be passed through dryers to remove water therefrom. Methanol can be removed from the alkylation stream by known processes, for example, by extraction with water. However, the removal of dimethyl ether from the alkylation stream cannot be done economically by use of distillation or a simple water wash.
Many references exist showing the removal of undesired components from a hydrocarbon stream. Thus, Brown et al in U.S. Pat. No. 3,846,088 disclose a process wherein an alcohol is reacted with a hydrocarbon stream containing a tertiary olefin to produce an ether; unreacted hydrocarbons are removed from the reaction product and the bottoms containing unreacted alcohol are washed with water to reduce the alcohol content thereof. U.S. Pat. No. 3,847,756 to Statman et al discloses a process for the purification of a stream containing diethyl ether which comprises feeding the same to a distillation column, introducing water at a higher level in the column and removing pure diethyl ether from the column just below the top of the column, while low-boiling impurities are removed from the top of the column and high-boiling impurities are removed from the bottom of the column. In U.S. Pat. No. 4,118,425 to Herbstman there is disclosed a process wherein a crude mixture containing ethers, such as the methyl ether of tertiary butanol, prepared by reacting methanol and isobutene in the presence of hydrocarbons, is subjected to extraction to remove unreacted excess methanol therefrom using water. Rao et al in U.S. Pat. No. 4,144,138 disclose a process wherein methyl tertiary butyl ether is recovered by azeotropic distillation to obtain a methanol-ether overhead azeotrope which is then water washed. The water washing results in a pure ether raffinate and an ether-methanol bottom product. The bottoms are then azeotropically distilled to give an ether-methanol overhead that is recycled to the water washing step. Chase et al in U.S. Pat. No. 4,218,569 relate to a process wherein an etherification crude product containing residual methanol is fractionally distilled to obtain a distillate containing methanol and hydrocarbons and the distillate is then contacted with a glycol to remove the methanol therefrom. The process defined in U.S. Pat. No. 4,302,298 to Mikitenko et al isolates methyl tertiary butyl ether in the reaction product of methanol with a C.sub.4 -hydrocarbon cut containing isobutene comprising fractionation to separate methyl tertiary butyl ether therefrom, the remainder of the reaction product containing C.sub.4 -hydrocarbons and methanol is water washed, a portion of the separated C.sub.4 -hydrocarbons from the water wash is recycled to the distillation zone and the water-methanol phase from the washing is distilled to separate the methanol therefrom. Prezelj et al in U.S. Pat. No. 4,334,964 relate to a process wherein an etherification stream is water washed to extract alcohol components therefrom, the extractant is distilled to recover methanol and a tertiary alcohol side stream is removed from the distillation zone. Herskovits in U.S. Pat. No. 4,465,870 states that he can use a solid regenerable sorbent to remove undesirable compounds such as water, methanol or ether, from a hydrocarbon recycle stream withdrawn from an etherification process. In U.S. Pat. No. 4,479,018 to Van Pool there is disclosed a process wherein a mixture of olefins containing isobutylene is reacted with methanol to form a product ether, the ether is removed from the total product and the remainder of the product containing unreacted olefins and unreacted methanol is washed with water to remove methanol therefrom.
None of the above references, alone or in combination, discloses or teaches the novel process herein, namely, the removal of dimethyl ether from a hydrocarbon mixture containing the same by contacting said hydrocarbon mixture with an aqueous solution containing a polar oxygenated hydrocarbon having a polarity of about 1.4 to about 2.0 Debyes specifically as defined herein.